1. Field of the Invention
This invention relates to the production of ammonium sulfate and calcium carbonate from gypsum obtained from flue gas desulfurization (FGD) systems located at electric power plants. While FGD gypsum provides a cheap source of gypsum, the physical structure of the gypsum crystalline particles may have characteristics that make it difficult to react with other chemicals. The process of the present invention overcomes this problem and other difficulties to result in an economically viable method to produce ammonium sulfate of high purity and yield.
FGD gypsum is a synthetic product resulting from sulfur dioxide (SO2) gas emission control systems used at fossil fuel and particularly, coal combustion power plants to remove sulfur from the combustion gases using “scrubber” devices. The sulfur dioxide is derived from any sulfur containing compounds in the fuels. A wet scrubber uses lime (calcium oxide or calcium hydroxide) or more typically, limestone (calcium carbonate) to react with sulfur dioxide gas to remove the sulfur in a solid form. The reaction in wet scrubbing uses a limestone (CaCO3)-water slurry to produce calcium sulfite (CaSO3) according to the following chemical reaction:CaCO3 (solid)+SO2 (gas)→CaSO3 (solid)+CO2 (gas)
In the above chemical reaction, the production of carbon dioxide (CO2) causes a potential detrimental release of carbon dioxide into the flue gas which may lead to violation of air quality regulations and possibly lead to a greater atmospheric greenhouse effect.
To partially offset the cost of the FGD installation, the CaSO3 (calcium sulfite) may be further oxidized (known as forced oxidation) to produce CaSO4.2H2O (FGD gypsum) according to the following chemical reaction:CaSO3 (solid)+H2O (liquid)+½O2 (gas)→CaSO4 (solid)+H2O Hydration CaSO4.½H2O+1½H2O→CaSO4.2H2O
FGD gypsum consists of small, fine, crystalline particles and is chemically nearly identical to mined natural gypsum. However, FGD gypsum produced at different power plants may differ slightly in chemical composition and in crystalline structure. Most chemical differences are due to impurities from the employed fuel. Structurally, however, some FGD gypsum may be composed of crystalline particles that have less surface area and are thus less reactive than other FGD gypsum crystalline particles. FGD crystals that are thicker and more spherical have less reactive surface area. Thus, a process that employs FGD gypsum as a starting material, must be able to accommodate the less reactive FGD gypsum particles as well as FGD gypsum crystalline particles resulting from other FGD installations that produce particles having flatter, more disc like structure with greater reactive surface area.
The process of the present invention employs a chemical reaction of FGD gypsum with ammonium carbonate ((NH4)2CO3) to produce ammonium sulfate ((NH4)2SO4) and calcium carbonate (CaCO3). Both the ammonium sulfate and calcium carbonate products are commercially valuable materials and are produced by the present process in high purity and high yield.
Ammonium sulfate (21-0-0-24S) is used most commonly as a chemical fertilizer for alkaline soils. When applied to damp soil, an ammonium ion is released which creates a small amount of acid, that lowers the pH balance of the soil. In the soil, the ammonium ions are converted to nitrate by soil bacteria which contributes nitrogen to the soil and aids in plant growth. Ammonium sulfate dissolves relatively slowly (ammonium sulfate—74.4 g/100 ml (20° C.), urea—107.9 g/100 ml (20° C.), ammonium nitrate—150 g/100 ml (20° C.)), which makes for more efficient use and thus reduces cost compared to some other artificial fertilizers.
Common nitrogen fertilizers include anhydrous ammonia (82% N), urea (46% N), urea and ammonium nitrate solutions (28-32% N), ammonium sulfate (21% N) and ammonium nitrate (34% N). Ammonium sulfate (21%) is a nitrogen source with little or no surface volatilization loss when applied to most soils. It is easy to store and is not as hygroscopic as ammonium nitrate. Ammonium sulfate is a good source of sulfur when it is needed to correct or prevent a sulfur deficiency. In areas with high pH soils, the sulfur in ammonium sulfate helps lower soil pH levels.
In addition to use as fertilizer, ammonium sulfate is used as an agricultural spray adjuvant for water soluble insecticides, herbicides and fungicides. In this capacity, it functions to bind iron and calcium cations that are present in both well water and plant cells. It is particularly effective as an adjuvant for 2,4-D (amine), glyphosate, and glufosinate herbicides.
Ammonium sulfate is used in flame retardant materials because it lowers the combustion temperature and increases the production of residues or chars.
In biochemistry, ammonium sulfate precipitation is a common method for purifying proteins by precipitation. As such, ammonium sulfate is also listed as an ingredient in many vaccines used in the United States. The DTap vaccine, which protects children from diphtheria, tetanus, and whooping cough, uses ammonium sulfate for this purpose.
Fine calcium carbonate results as precipitated particles from the process of the present invention and is useful in many industries.
High purity calcium carbonate is used as dietary calcium supplement to help ensure healthy bones and teeth. Calcium carbonate supplement is effective to treat certain medical disorders related to calcium deficiency such as osteoporosis and to reduce acid in the stomach and relieve indigestion and heartburn. For irritable bowel syndrome, a calcium carbonate supplement may be taken to reduce or relieve diarrhea. Calcium carbonate is used in the production of toothpaste and as an inert substance in pharmaceutical or dietary supplement tablets.
Fine calcium carbonate is the most preferred mineral in the paper industry, used for filling and coating paper. It helps in production of the best quality printing papers. Precipitated calcium carbonate is used as a filler in paper because it is cheaper than wood fiber wherein printing and writing paper can contain 10-20% calcium carbonate. In North America, calcium carbonate has begun to replace kaolin in the production of glossy paper. Europe has been practicing this as alkaline or acid-free papermaking for several decades. Precipitated calcium carbonate is especially useful compared to ground calcium carbonate because of having a very fine and controlled particle size, on the order of 2 micrometers in diameter, which is of particular utility in producing coatings for paper.
In the oil industry, calcium carbonate is added to drilling fluids as a formation-bridging and filter cake sealing agent and can also be used as a weighting material to increase the density of drilling fluids to control the down-hole pressure.
Additionally, with respect to the above described production of carbon dioxide by scrubbing the flue gas which ultimately produces FGD gypsum, the ammonium carbonate used in the process of the present invention will be produced by removing the carbon dioxide created by the scrubber and reacting it with ammonia in a separate reactor and process to produce the ammonium carbonate. Thus, a complete recycle of the scrubber carbon dioxide will be achieved by employing it in the form of ammonium carbonate to feed back into the process of the present invention. This would help lead to compliance with air quality regulations and possibly lead to less atmospheric greenhouse effect.
2. Description of Related Art
Because of increasingly stringent flue gas environmental standards for coal fired, electric power plants, the amount of solid waste, such as FGD gypsum, generated by flue gas scrubbers has increased to a very large volume. While others have previously attempted to make useful products from the FGD gypsum or similar gypsum waste products, they have failed to employ processes that can produce ammonium sulfate and calcium carbonate of the high purity and efficiency of yield provided by the process of the present invention.
For example, the publication, WO 2005/11822A discloses a process for treating phosphogypsum that produces calcium carbonate and ammonium sulfate, but the calcium carbonate is impure due to the process failure to include a procedure to remove ammonium sulfate or ammonium carbonate from the produced calcium carbonate. The disclosed process also fails to include procedures for purifying the incoming gypsum feedstock.
In the British patent, GB 437,278, a process is disclosed for the treatment of combustion and distillation gases from gas works and coke ovens by contacting the gases with calcium carbonate to recover calcium sulfate (gypsum) and react it with ammonia and carbon dioxide from the gases to make ammonium sulfate and calcium carbonate. However, the produced calcium carbonate is contaminated with calcium sulfate and the conversion of reactants to products is incomplete.
The German patent no. 610786 discloses a process for continuous conversion of calcium sulfate and ammonium carbonate to ammonium sulfate and calcium carbonate. This patent shows the difficulty in obtaining reaction products, such as calcium carbonate of high purity due to the difficulty of separating the small precipitated crystals of calcium carbonate from the ammonium sulfate. In this process, seed crystals of calcium carbonate are added to the produced ammonium sulfate solution to grow large crystals of calcium carbonate which can be more effectively filtered and removed from the ammonium sulfate product.
German patent no. 612806 disclosed a process for continuous conversion of calcium sulfate and ammonium carbonate to ammonium sulfate and calcium carbonate. Significantly, the disclosed process is co-current and not countercurrent as in one embodiment of the present invention and the time required for conversion is about five hours which is much slower than the conversion by the present countercurrent process which is about ten minutes. While the disclosed conversion yield is 97%, the conversion yield of the present countercurrent process is as high as approximately 100%.